Ion beam technology is actively practiced in many industrial fields. Many types of ion sources have been developed for generating ions of various elements with high efficiency. Among them is an electro-hydrodynamic (EHD) ion source that uses a pure metal or alloy in a molten state as the ion material with the ions being extracted under a high electric field. This is also called a liquid metal ion source (LMIS) because it is based on the technique of extracting ions from a metal in a molten state with its ion emission being performed in a high electric field.
Liquid metal ion sources are known to have high brightness since ions are emitted from a point area of the emitter. The ion stream that is emitted can be focused into a beam (generally referred to as a focused ion beam or FIB) of a diameter less than 1 .mu.m and projected onto a specimen as a final target, for example. This type of FIB has a high-current-density and extremely-fine beam, and so ion lithography, ion implantation, etching and the like can be implemented using this FIB in semiconductor processes without the need for conventional masks, that is, in a maskless manner. Further, such an FIB is also useful in secondary ion mass spectrometry wherein a specimen is irradiated with an ion beam to generate secondary ions that are expelled from the specimen by sputtering for analysis. Since the beam is of a very small diameter, the FIB can be used to carry out component analysis in a sub micron region on the surface of the specimen. Accordingly, FIB applications that rely on an LMIS are being made in various fields.
The structure of an LMIS and its principle of operation will be explained with reference to FIG. 2 and the following description. A typical LMIS, for example, as described in "Development of Boron Liquid Metal Ion Source" (hereinafter Prior art 1) by T. Ishitani et al., Journal of Vacuum Science and Technology A2, (1984) pp. 1365-1369, has an ion material 1 to be ionized and a heater 2 for retaining the material 1 in a liquid or molten state. An emitter 4 is disposed such that the ion material 1 is supplied from the heater 2 to the emitter for emitting ions 3 from the emitter apex. An extractor electrode 5 is provided for concentrating a high electric field around the emitter apex for extracting ions 3. The apparatus is enclosed in a vacuum chamber 6 having electric feed through terminals 7 and 7', and power supply units 8, 8' and 8". As required, the material to be ionized is maintained in a liquid or molten condition by methods such as resistance heating by current conduction of a reservoir member for holding the ion material, electron bombardment heating in the vicinity of the emitter apex, heating with a heater wound around the reservoir that retains the ion material in a liquid or molten state, and the like. These basic constructions of an LMIS, however, are not greatly different from one another.
An LMIS having such an arrangement as above operates as follows. After evacuation of the vacuum chamber 6, the heater 2, which also serves as the reservoir, heats the ion material. Consequently, the ion material 1 and emitter 4 are heated by thermal conduction, and the liquid or molten ion material 1 is supplied to the top of the emitter 4 by wet spreading along the surface. Then, when the extractor electrode 5 is maintained with a high negative voltage, an electrical field is concentrated around the emitter apex. By applying still a higher voltage, the molten metal forms a conical protrusion, called a Taylor cone. Once a certain threshold voltage is reached, ions 3 are extracted from the emitter apex. The extracted ions pass through an ion optical system (not shown) having lenses, deflectors and the like that is disposed on the downstream side of the ion source to form an FIB.
The most widely used types of an LMIS are the hairpin type and reservoir type. FIG. 3(a) illustrates a hairpin type LMIS wherein a needle electrode 13 is spot-welded to the center portion of a fine wire 12 that is connected between two electric feed through terminals 11, 11' that protrude through an insulator base plate 10. An example of this type is disclosed in the Japanese Patent Publication No. 3579/1983 (hereinafter Prior art 2). This type of LMIS has a very simple construction. Wire 12 serves as a reservoir to store the ion material 14, and also serves as a heater through which a current passes between lead-in terminals 11 and 11'.
FIG. 3(b) shows a reservoir type LMIS. Two electric feed through terminals 16, 16' extend through an insulator base plate 15 and are secured thereto. A reservoir 18 stores an ion material 17 and is supported by wires 19, 19' which conduct a heating current to the reservoir 18. Further, an emitter 20 is fixed to the reservoir 18. An advantage of this type of LMIS is that a large amount of ion material 17 can be contained in the reservoir. An example of this type has been disclosed in Japanese Patent Publication No. 38905/1983 (hereinafter Prior Art 3).
As a modification of the reservoir type LMIS, there is a capillary needle type LMIS, as shown in FIG. 3(c). The bottom of the reservoir 21 is a capillary 22. The space between the emitter 23 and the capillary 22 is very small and accordingly the molten ion material 24 flows to the emitter apex 23 after passing through the space due to capillary action.
In each of the foregoing types, the LMIS is designed to be quickly disconnected from an ion beam apparatus, and thus is provided in a cartridge that includes electric feed through terminals, an emitter, a reservoir and an insulator, which can be readily replaced when the ion material is exhausted or the needle electrode is damaged and the like. Namely, this type of LMIS to which the present invention is directed is of a so-called cartridge type.
It is mentioned in the prior art that cleaning of the reservoir and emitter is important for ensuring the efficient and stable operation of an LMIS. For example, a combined prior art high temperature cleaning method for cleaning the reservoir and emitter, and subsequent method for charging the emitter and reservoir by dipping them into a molten ion material have been described in a paper titled "Liquid Gold Ion Source" by A. Wagner et al., Journal of Vacuum Science and Technology (1979) Vol. 16 pp. 1871-1875 (Prior art 4). According to this method, as shown in FIG. 4(a), an ion material 32 is stored in a melting pot 31 heated by a filament 30.
After the ion material 32 becomes molten, a filament type LMIS 33 is heated by current conduction and dipped into the ion material 32 such that the molten ion material 32 is allowed to adhere to an emitter 34 and a heater 35. FIG. 4(b) shows LMIS 37 after immersion with the ion material 36 adhered to the emitter and heater.